The production of polyurethane resins is a well known commercial process as described, for example, in the Kirk-Othmer, Encyclopedia of Chemical Technology, First Supplement, pages 888 et sec., (Interscience, 1957). Briefly, these polymers are prepared by means of the reaction between an isocyanate, such as toluene diisocyanate or diphenylmethane-4-4'diisocyanate, and a second reagent comprising a polyol which contains two or more hydroxyl groups. As used in this specification, the term "isocyanate material" is intended to include any polyisocyanate or urethane compounds containing two or more unreacted-NCO radicals.
The most common polyurethanes are formed by reaction of toluene diisocyanate and a polyol or polyester. Representative polyesters are the reaction products of adipic acid and/or phthalic anhydride and ethylene glycol. Other polyols which may be used in place of the polyesters are polyethers, such as the polyoxypropylenediols and polyoxypropylene triols, caster oil and drying oils, etc.
Polyurethane foams, or urethane foams as they are frequently referred to, differ from other cellular plastics groups in that the chemical reactions causing foaming occur simultaneously with the polymer-forming reactions. As in the case of polyurethane resins, the polymeric constituent of urethane foams is made by reacting a polyol with an isocyanate. When the isocyanate is in excess of the amount that will react with the polyol, and when water is present, the excess isocyanate will react with water to produce carbon dioxide which expands the mixture. Urethane foams may be flexible or rigid and may have open or closed cells, depending largely on the polyol used. Crosslinked foams are rigid or semi-rigid. Auxiliary blowing or foaming agents, such as the various halohydrocarbons, are sometimes used especially in rigid foams. Other ingredients often incorporated in urethane foams are catalysts to control the speed of reaction, and a surfactant to stabilize the rising foam and control cell size.
Three basic processes are used for making urethane foams: The prepolymer technique, the semi-prepolymer technique and the one-shot process. In the prepolymer technique, a polyol and an isocyanate are reacted to produce a compound which may be stored and subsequently mixed with water, catalyst and, in some cases, a foam stabilizer. In the semi-prepolymer process, about 20% of the polyol is prereacted with all of the isocyanate, then this product is later reacted with a masterbatch containing the remainder of the ingredients. And, in the one-shot process, an isocyanate, a polyol and catalyst are fed into separate streams to a mixing head from which the mixed reactants are discharged into a mold.
Regardless of the procedure utilized for their preparation, polyurethane foams are enjoying ever increasing utilization in a wide variety of applications including their use in household appliances, airplane construction, padding for mattresses and upholstery, interlinings for overcoats and sleeping bags, soundproof walls, insulation against heat loss, life preservers, fish net floats, foam rubber specialties, air filters, packaging and bone surgery.
In many of the above listed applications for polyurethane foams, they are utilized under conditions where they may be exposed to fire or to prolonged high temperatures. Accordingly, it may be necessary or desirable that the foam have flame retardant properties so that burning will be eliminated or minimized as by having the burning rate reduced in the event that it should catch fire. Various materials have been incorporated into polyurethane foams in order to provide them with such flame retardant properties. These additives may be divided into two broad classes. The first of these comprise simple, extraneous additives which do not react with any of the components of the foam. Exemplary of such extraneous additives are the halogenated hydrocarbons and halogen substituted, phosphates or polyphosphonates.
The second broad class of flame retardant additives for polyurethane foams comprise the so-called functional additives which have reactive or functional groups capable of reacting with the polyol or isocyanate reagents used in preparing the foams. Accordingly, when admixed with the foam forming ingredients, such compounds become chemically incorporated as an integral component of the resulting foam. Since it is an integral part of the foam, it cannot be readily removed therefrom as by solvent leaching or other means such as may be prone to affect the loss of those extraneously added materials which are merely physically blended with the foam.
One such group of functional or reactive flame retardant additives are the polyglycol hydrogen polyphosphonates of the formula: ##EQU1## wherein R is a polyoxyalkylene ether radical and n is an integer having a value of from 1 to 5. These compounds are described and an improved method for their preparation is claimed in application Ser. No. 709,952, filed Mar. 4, 1968, and now abandoned, the entire disclosure of which is here incorporated by reference.
Although these polyglycol hydrogen polyphosphonates have provided excellent results as reactive flame retardant additives for polyurethane foams, there have been some problems associated with their use. Thus, for example, when introduced in a high concentration of from about 10 to 30%, by weight, in order to yield foams displaying a particularly high degree of flame retardancy, it has been found that some of the physical properties of the resulting foams, such as their density and resiliency, are somewhat deficient as compared with those of comparable foams which do not contain these reactive additives.
Thus, it is the prime object of this invention to provide a means for employing high concentrations of polyglycol hydrogen polyphosphonates as flame retardant additives for flexible polyurethane foams without adversely affecting the physical properties of the resulting foams. A further object involves the preparation of flexible polyurethane foams characterized by their unusual combination of outstanding flame retardancy along with their excellent physical properties. Various other objects and advantages of this invention will be apparent from a reading of the disclosure which follows hereinafter.